In general, a touch panel is a device that is installed on the surface of a display device and converts physical contact such as a finger of a user, a touch pen, and the like into an electrical signal, and is applied for a liquid crystal display, a plasma display panel, an EL (electro-luminescence) device, and the like.
Such a touch panel is commercialized as an optical type using infrared rays, a transparent electrode type using a contact point of a transparent conductive film formed by coating a metal oxide on a polymer substrate film, a capacitance type detecting a change in capacity, a position sensitive type that senses a position from the distribution of force to a pressure sensor, and the like.
Among the various types of touch panels, a transparent electrode type requires a transparent metal oxide-based conductive film, and an ITO film formed by coating indium tin oxide (ITO) on a polymer substrate film is widely used because it is transparent and has excellent conductivity.
Particularly, the metal oxide-based conductive film may be prepared by vacuum deposition or sputtering a metal oxide on a polymer substrate film or a glass substrate. Thus, heat resistance is one of essential properties of a base film, because processes such as vacuum deposition, sputtering, and the like that are progressed at a high temperature should be conducted.
In general, heat resistance may be assessed by glass transition temperature, and to obtain a base film on which an optically excellent transparent metal oxide-based conductive film is formed, sputtering at a temperature of equal to or greater than about 150° C., preferably at a temperature of equal to or greater than about 200° C., is required, and thus a high glass transition temperature above the sputtering temperature and excellent mechanical properties are required for the base film.
Further, in order to process the base film and apply it to a display device and the like, transparency in the visible light region is also an important property in addition to the above-explained high glass transition temperature, and light transmittance of 90% or more is required.
However, a polymer film such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), and the like, which is conventionally used as a base film, has a low glass transition temperature of 120° C. or less, thus generating a problem of deformation in the process of forming a transparent metal oxide-based conductive film. In addition, a polyimide (PI) film, on which many studies have recently being progressed due to its excellent heat resistance and high glass transition temperature of 400° C. or more, but is not transparent and shows an orange color, and thus is not suitable for application in the display field.
Further, since a polymer film such as polyethylene terephthalate (PET) and the like has a large phase difference, it can be used as an external base film, but cannot be used as an internal base film.
Accordingly, there is a demand for development of a base film that has properties of a high glass transition temperature, heat resistance, and the like, has small a phase difference, and is transparent, a laminated structure including the same, and a display device including the laminated structure.